1. Field of the Invention
This invention relates to an electrophotographic apparatus, and more particularly to an electrophotographic apparatus having a specific electrophotographic photosensitive member and a specific transfer means.
2. Related Background Art
Inorganic materials such as zinc oxide, selenium, and cadmium sulfide are hitherto known as photoconductive materials used in electrophotographic photosensitive members. Organic materials including polyvinyl carbazole, phthalocyanine and azo pigments have attracted notice based on the advantages that they promise high productivity and are free from environmental pollution, and have been put into wide use although they tend to be inferior to the inorganic materials in respect of photoconductive performance or running performance. In recent years, new materials having overcome such disadvantages have been studied, and are surpassing the inorganic materials particularly with regard to photoconductive performance.
Meanwhile, electrophotographic photosensitive members are required to have various chemical and physical durabilities since they are repeatedly affected by charging, exposure, development, transfer, cleaning and charge elimination in electrophotographic processes in copying machines or laser beam printers. In particular, surface properties of photosensitive members, such as surface energy, are concerned in developer transfer performance on photosensitive members, contamination of photosensitive members and so forth, and are one of the important factors for obtaining high-quality images. Most of the above organic photoconductive materials have no film forming properties by themselves, and hence they are commonly formed into films in combination with binder resins or the like when photosensitive layers are formed. Accordingly, properties of :such binder resins can be referred to as a factor that greatly influences the surface properties such as surface energy.
Binder resins conventionally used include polyester, polyurethane, polyarylate, polyethylene, polystyrene, polybutadiene, polycarbonate, polyamide, polypropylene, polyimide, polyamidoimide, polysulfone, polyallyl ether, polyacetal, nylon, phenol resins, acrylic resins, silicone resins, epoxy resins, urea resins, allyl resins, alkyd resins and butyral resins. However, those having better surface properties are being studied.
Incidentally, in recent years, there is a demand for electrophotographic processes that can faithfully reproduce color images, and several systems have been proposed. Among them, apparatus employing a multiple-transfer system are commonly available, in which a photosensitive drum and a transfer drum that carries a transfer material such as transfer paper are synchronized drum-to-drum and images corresponding to the three primary colors or four colors comprised of these three colors and a black color added thereto are successively superimposed on the transfer material to reproduce a color image.
One of the problems involved in such a process, is the transfer efficiency of the second and subsequent colors at the time of multiple transfer has been questioned. More specifically, the transfer of the second and subsequent colors is carried out via a developer which has already been transferred to a transfer material, and hence such transfer can only more indirectly operate than usual transfer. As a result, the developer which has not been transferred and is standing on the photosensitive member can not be transferred to the side of the transfer material, so only low-quality images can be obtained because of faulty transfer. Especially when the aforesaid conventional organic photosensitive members are used, faulty copying such as uneven transfer at solid image areas or letter blank areas caused by poor transfer tends to occur.
As another problem, the driving load of photosensitive members has been questioned. In particular, the step of cleaning to remove the developer remaining on the photosensitive member after transfer has a great influence on the driving load. As a cleaning method, blade cleaning should be employed so that the construction of the apparatus can be made simpler and more effective and the space for the apparatus can be saved. Blade cleaning usually takes a simple construction in which a platelike elastic member made of polyurethane or the like is brought into push contact with the surface of the photosensitive member in the direction of its generatrix. In the case when, however, the aforesaid conventional organic photosensitive members are used, a great contact energy is produced between the photosensitive member and the blade, so that a heavy load is applied to the driving of the photosensitive member. As a result, a disturbance such as uneven drive may occur in the driving of the photosensitive member which causes color misregistration wherein images corresponding to the second and subsequent colors are misregistered at the time of multiple transfer, or faulty copying such as drive pitch unevenness wherein the uneven drive comes out as an uneven image density. In particular, in apparatus in which as a light source for forming a latent image a laser, an LED or a liquid crystal shutter is used to form a dotlike minute latent image, the color misregistration on the micron order may easily occur unless the dots are superimposed at a high precision at the time of multiple transfer, to cause aberration of color tones, a decrease in image sharpness, etc.